Millions of mature trees died last year in California, adding to the browning of the state’s forests and open spaces while providing fuel for the next round of wildfires that most assuredly will hit the state when warmer temperatures and hot winds return, federal officials said Monday.

The Holy fire burns in the mountains in the Cleveland National Forest near Glen Ivy Hot Springs in Corona on Saturday, August 11, 2018. Dead trees can contribute to hotter and more intense fires.<br />(Photo by Mark Rightmire, Orange County Register/SCNG)

A new survey released by the U.S. Forest Service found an additional 18.6 million trees have died since fall 2017.

That brings the mortality total to 147.6 million trees across about 10 million acres of federal, state and local lands since the drought began in 2010.

The recent tree deaths makes the cumulative tree die-off one of the largest in the history of the state, said Sheri Smith, USFS entomologist.

Slowing death numbers

While the above-average rainfall in winter 2016-2017 ended the drought, the return to below-average amounts of precipitation in 2017-2018 killed off more trees, according to the study.

However, the number of trees dying each year have slowed since the drought years, a sign that rain has helped some trees recover.

In 2018, surveyors detected the lowest number of additional dead trees since 2014.

“It is going down. But trees are still dying,” said Stephanie Gomes, USDA Forest Service information officer.

The survey found 42,000 dead trees in the San Bernardino National Forest; 4,000 in the Cleveland National Forest and 9,000 in the Angeles National Forest, Smith said.

Thom Porter, CalFire director, said the slowing of the rate of dead trees is a positive, but the state still faces extreme fire danger from the withered trees still standing in dense brush and in forests and hillsides surrounding homes.

San Gorgonio wilderness volunteers, Ed Nemeth, Cliff Heck and Jeremy Dorsey of the U.S. Forest Service, left to right, move a burned tree trunk as they do trail repair work in 2016. (Photo by Kurt Miller, THE PRESS-ENTERPRISE/SCNG)

“Eighteen million trees are an indication that the forests of California are still under significant stress,” Porter said in a statement. “The stress of drought, insects, disease, and prolific wildfire will continue to challenge the resilience of the state’s forests.”

Wildfire fuel

State and federal resource management agencies are in a race to remove dead trees and thin out dense brush to lessen the likelihood of fast-spreading wildfires, such as those experienced in the last two years.

The December 2017 Thomas Fire in Ventura and Santa Barbara counties burned 281,893 acres and the Woolsey Fire last fall in Thousand Oaks and Malibu killed three people, destroyed 1,643 structures and burned 96,949 acres.

Since 2016, state, federal and local agencies have chopped down 1.5 million dead trees, the Forest Service reported.

In Southern California, the survey found dead white fir, Jeffrey, Coulter and ponderosa pine trees clustered in 5,900 acres of the San Bernardino, Angeles and Cleveland forests. The survey found “light to moderate” amounts of dead pine trees in the northern section of the San Gorgonio Wilderness, located north of Yucaipa.

Oak trees, usually more resilient to insects and drought, were dying near the southern end of Lake Henshaw, at the southeast base of Palomar Mountain, in and near the Cleveland National Forest.

Thinning of forest lands can make healthy trees more resilient to bark beetles and the goldspotted oak borer. Both pests infest trees until they can no longer take up nutrients.

The fight against invading pests is a never-ending challenge, made worse by too many trees competing for too little water, Smith said.

The Forest Service completed 313,000 acres of forest restoration last year. That includes about 63,000 acres thinned by prescribed burns, the largest numbers since 2001.

Most dead trees in 2018 were found clustered in the Southern Sierra Nevada, stretching from Bishop to Bakersfield. “Moderate” to “Very Severe” tree die-offs have occurred south of Yosemite National Park, including Sequoia & Kings Canyon National Parks and farther south into the forests of Inyo County, aerial maps show.

Steve Scauzillo covers environment and transportation for the Southern California News Group. He has won two journalist of the year awards from the Angeles Chapter of the Sierra Club and is a recipient of the Aldo Leopold Award for Distinguished Editorial Writing on environmental issues. Steve studied biology/chemistry when attending East Meadow High School and Nassau College in New York (he actually loved botany!) and then majored in social ecology at UCI until switching to journalism. He also earned a master's degree in media from Cal State Fullerton. He has been an adjunct professor since 2005. Steve likes to take the train, subway and bicycle – sometimes all three – to assignments and the newsroom. He is married to Karen E. Klein, a former journalist with Los Angeles Daily News, L.A. Times, Bloomberg and the San Fernando Valley Business Journal and now vice president of content management for a bank. They have two grown sons, Andy and Matthew. They live in Pasadena. Steve recently watched all of “Star Trek” the remastered original season one on Amazon, so he has an inner nerd.